Along with surgical therapy and radiation therapy, cancer chemotherapy plays an important role in “treatments for cancer.” Starting with Nitrogen Mustard-related compounds which were clinically used as antitumor agents in 1940s, various types of antitumor agents have been developed for approximately sixty years since then. However, the antitumor agents clinically used have problems including side effects and the appearance of cells with acquired resistance. The biological activity of chemical substances depends largely on their chemical structures. Accordingly, there has been a growing demand for a novel compound having antitumor activity, which is suitable for an antitumor agent.